Description:
------------
This seemingly happens with datetime objects only, not with simple user defined objects. Reproducible with PHP 7.4 Alpha 1 but not with PHP 7.3.6

Test script:
---------------
<?php
class foo
{
    public $a;
    public $b;
    
}

$bar = new foo;
$bar->a = 'A';
$bar->b = 'B';

var_dump($bar);

$arr = (array) $bar;

var_dump($arr);

$date = date_create('2018-12-31 23:59:59.997');

date_default_timezone_set("America/Los_Angeles");

var_dump($date);

$darr = (array) $date;

var_dump($darr);

?>

Expected result:
----------------
object(foo)#1 (2) {
  ["a"]=>
  string(1) "A"
  ["b"]=>
  string(1) "B"
}
array(2) {
  ["a"]=>
  string(1) "A"
  ["b"]=>
  string(1) "B"
}
object(DateTime)#2 (3) {
  ["date"]=>
  string(26) "2018-12-31 23:59:59.997000"
  ["timezone_type"]=>
  int(3)
  ["timezone"]=>
  string(3) "UTC"
}
array(3) {
  ["date"]=>
  string(26) "2018-12-31 23:59:59.997000"
  ["timezone_type"]=>
  int(3)
  ["timezone"]=>
  string(3) "UTC"
}


Actual result:
--------------
object(foo)#1 (2) {
  ["a"]=>
  string(1) "A"
  ["b"]=>
  string(1) "B"
}
array(2) {
  ["a"]=>
  string(1) "A"
  ["b"]=>
  string(1) "B"
}
object(DateTime)#2 (3) {
  ["date"]=>
  string(26) "2018-12-31 23:59:59.997000"
  ["timezone_type"]=>
  int(3)
  ["timezone"]=>
  string(3) "UTC"
}
array(0) {
}